The present invention relates to novel modified alginic acid or alginic acid derivatives and anti-reflective coating compositions thereof and their use in image processing by forming a thin layer between a reflective substrate and a photoresist coating. Such compositions are especially useful in the fabrication of semiconductor devices by photolithographic techniques.
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has lead to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization. The use of highly absorbing anti-reflective coatings in photolithography is a simpler approach to diminish the problems that result from back reflection of light from highly reflective substrates. Two deleterious effects of back reflectivity are thin film interference and reflective notching. Thin film interference results in changes in critical linewidth dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist changes. Variations of linewidth are proportional to the swing ratio (S) and therefore must be minimized for better linewidth control. Swing ratio is defined asS=4(R1R2)1/2e−αDwhere R1 is the reflectivity at the resist/air or resist/top coat interface,where R2 is the reflectivity at the resist/substrate interface,where α is the resist optical absorption coefficient, andD is the film thickness.
Bottom anti-reflective coatings function by absorbing the radiation used for exposing the photoresist, thus reducing R2 and thereby reducing the swing ratio. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which scatter light through the photoresist film, leading to linewidth variations, and in the extreme case, forming regions with complete resist loss. Similarly, dyed top anti-reflective coatings reduce the swing ratio by reducing R1, where the coating has the optimal values for refractive index and absorption characteristics, such as absorbing wavelength and intensity.
In the past, dyed photoresists have been utilized to solve these reflectivity problems. However, it is generally known that dyed resists only reduce reflectivity from the substrate but do not substantially eliminate it. In addition, dyed resists also cause reduction in the lithographic performance of the photoresist, together with possible sublimation of the dye and incompatibility of the dye in resist films. In cases where further reduction or elimination of the swing ratio is required, the use of a bottom anti-reflective coating provides the best solution for reflectivity. The bottom anti-reflective coating is applied to the substrate prior to coating with the photoresist and prior to exposure. The resist is exposed image-wise and developed. The anti-reflective coating in the exposed area is then etched, typically in an oxygen plasma, and the resist pattern is thus transferred to the substrate. The etch rate of the anti-reflective film should be relatively high in comparison to the photoresist so that the anti-reflective film is etched without excessive loss of the resist film during the etch process.
Anti-reflective coatings containing a dye for absorption of the light and an organic polymer to give coating properties are known. However, the possibility of sublimation and diffusion of the dye into the environment and into the photoresist layer during heating make these types of anti-reflective compositions undesirable.
Polymeric organic anti-reflective coatings are known in the art but are typically cast from organic solvents, such as cyclohexanone and cyclopentanone. The potential hazards of working with such organic solvents, have lead to the development of the anti-reflective coating composition of the instant invention, where the solid components of the anti-reflective coating are both soluble and spin castable from solvents having lesser toxicity hazards. However, the novel dye functionality of the instant invention when attached to the specific types of monomer described, makes the instant invention significantly different from the prior art referred to previously. Another advantage of using anti-reflective coatings soluble in lower toxicity solvents is that these same solvents can also be used to remove the edge bead of the anti-reflective coating and no additional hazards or equipment expense is incurred, since these solvents are also used for photoresist and photoresist processing. The anti-reflective coating composition also has good solution stability. Additionally, substantially no intermixing is present between the anti-reflective coating and the photoresist film. The anti-reflective coatings also has good dry etching properties, which enable a good image transfer from the resist to the substrate and good absorption characteristics to prevent reflective notching and linewidth variations.
Various dye-attached, thermosetting binder chemistries have been developed. They include phenolic binders such as those described in U.S. Pat. No. 5,597,868 to Kunz; acrylic binders such as those described in European Patent Published Application EP 636941 to Urano et al., European Patent Published Application EP 542008 to Thackeray, U.S. Pat. Nos. 5,652,297 and 5,652,297 to McCulloch et al., and U.S. Pat. No. 5,919,599 to Meador et al.; modified epoxy resin binders such as those described in U.S. Pat. No. 5,693,691 to Flaim et al.; aliphatic polyester binders such as those described in U.S. Pat. No. 5,935,760 to Shao et al.; polysilane binders such as those described in U.S. Pat. No. 5,401,614 to Dichiara et al.; and vinyl aromatic binders such as those described in U.S. Pat. No. 5,482,817 to Dichiara et al., all of which are incorporated herein by reference. A recent disclosure describes thermosetting anti-reflective coatings derived from cellulosic binders. See U.S. Pat. No. 6,316,160. Alkylene glycol esters of alginic acid, described as useful emulsifying agents in oil in water emulsions, are described in GB 676,618. Esters of alginic acid, described where all or only some (90 to 100 percent) of the carboxylic groups of the acid are esterified, and the salts of the partial esters with metals or organic bases which are acceptable from a pharmacological point of view are described in EP 251905.